1. Field of the Invention
The present invention relates to an RD converter that converts resolver signals output from a resolver that detects a rotational angle of a motor into a digital output angle, and an angle detecting apparatus provided with the RD converter.
2. Description of the Related Art
In general, a resolver has an angle error, and the angle error has to be corrected in order to achieve precise angle detection.
A method of correcting the angle error of the resolver is described in Patent literature 1. According to the method described in Patent literature 1, angle error characteristics of the resolver are previously calculated based on comparison between output angles of the RD converter obtained by rotating the resolver at a constant number of revolutions and an angle data reference determined based on time measurement of the rotation of the resolver and recorded in a correction memory. The angle error characteristics are recorded in the correction memory in the form of corrected angles for the output angles of the RD converter.    Patent literature 1: Japanese Patent Application Laid-Open No. H10-170531
In operation, the output angle of the RD converter is input to the correction memory, the corrected angle associated with the output angle is output from the correction memory, and thus, an angle corrected for the angle error is acquired.
As described above, according to the method described in Patent literature 1, the angle error of the resolver is corrected by correcting the output angle of the RD converter. However, the characteristics of the angle error of the resolver having passed through the RD converter vary depending on the number of revolutions of the resolver, and the correction method described in Patent literature 1 is not designed for the angle error that varies depending on the number of revolutions and therefore cannot correct the angle error.
In addition, since the correction memory has to store the corrected angles and thus requires a high capacity. For example, when the resolution of the output angle of the RD converter is 12 bits, the correction memory has to have a memory capacity of 49152 bits (212×12 bits), because the memory has to store the corrected angle for each output angle.